As a rare earth magnet, for example, Sm—Co5 type, Sm2—Co17 type, Sm—Fe—N type, or R—Fe—B type (R indicates a rare earth element) is known, each of which is used as a high-performance permanent magnet. Among these, R—Fe—B type rare earth magnet is especially arousing interest because it employs mainly, as a rare earth element, neodymium (Nd) that occurs in great abundance and is relatively less expensive compared to samarium (Sm), and iron (Fe) is less expensive as well, and additionally because it has magnetic performance equivalent to or higher than Sm—Co type one.
However, the R—Fe—B type rare earth magnet contains, as the chief ingredient, a rare earth element and iron that are susceptible to oxidation and hence the corrosion resistance is relatively low, so that performance degradation and variations remain problems.
With the purpose of improving such a low corrosion resistance of the rare earth magnet, there has been proposed to form on the surface a protective film composed of an oxidation resistant metal or the like. For example, Patent document 1 describes the protective film in which two plated layers of nickel (Ni) are laminated, and Patent document 2 describes one in which a plated layer of nickel-sulfur (S) alloy is laminated on a plated layer of nickel.
Patent document 1: Japanese Patent No. 2599753
Patent document 2: Japanese Laid-Open Patent Publication No. H07-106109